Ataxia-telangiectasia (AT) is an autosomal recessive disorder characterized by oculocutaneous telangiectasias and progressive neuromotor dysfunction, cellular and humoral immune deficiencies, hypersensiticvity to ionizing radiation and increased predisposition to leukemias and lymphomas. The AT gene (ATM) encodes a 350 kD protein that acts as a serine/thereonine kinase by activating p53, a key molecule involved in the regulation of apoptosis and cell cycle. Antibodies generated against ATM protein are being used to study the other functional domains of the protein. In addition, an ATM DNA chip was developed and tested in order to faciliate the identification of carriers for the mutation. This will allow the testing of the link between carrier status and increased susceptibility for various malgnancies, including breast cancer.